The invention relates to heat-curable polysiloxane compositions and, in particular, to heat curable polysiloxane compositions containing asbestos therein.
In prior developments, asbestos has been used as a filler in polysiloxane compositions which were then cured to produce a silicone elastomeric composition used for a variety of purposes. In these disclosures of the prior art, asbestos was not distinguished from other inorganic fillers such as calcium carbonate, and not shown to produce any new properties in the resulting composition, but only shown to add stiffness or hardness to the resulting product. However, as mentioned earlier, from the prior art it is seen that asbestos fibers, when used as a filler in polysiloxane compositions, produced on the whole inferior types of silicone rubber as compared to the silicone rubber prepared by the use of high reinforcing silica fillers. On the other hand, it was desired to use asbestos fibers as fillers in polysiloxane compositions if a polysiloxane rubber composition could be produced with high tensile strength, high tear strength, high stiffness, which compositions were still flexible and had desirable per cent of elongation, since asbestos is very cheap.
It was further desirable to obtain a polysiloxane elastomeric composition which had high heat resistance and a good service life at both high and low temperatures within the temperature range of minus 40.degree.F up to and above 400.degree.F. Thus, in automobiles it is desirable to have gaskets which will have high strength and remain flexible for long periods of times at temperatures as high as 400.degree.F. On the other hand, it is also desirable that gaskets which often act as seals do not become brittle at low temperatures such as -40.degree.F to which the mechanisms of the automobile may be exposed. It is also desirable in this respect that hoses formed from a polysiloxane composition have a high tensile strength and which retain their strength at high temperatures, as well as at very low temperatures.
Another use for elastomers which have high heat resistance, as well as remain flexible at low temperatures, is to cover electrical wires and electrical components. Since such electrical wires and electrical components are normally exposed to very high temperatures, as well as low temperatures, it is required that the material covering and protecting the wires does not degrade in strength or become brittle at high or low operating temperatures. It is also desired to have polysiloxane elastomers with high tensile strength and stiffness or hardness, high tear strength, good abrasion resistance, flexibility and the desired elongation so that such material could be used as a covering material. The present polysiloxane elastomeric compositions are sometimes lacking in sufficiently high tensile strength and hardness. To produce such polysiloxane elastomeric composition having the above properties which could be put to the uses mentioned above, it was suggested that different fillers be used or incorporated into the polysiloxane composition so that the resulting composition will have the desired properties. However, all approaches or uses of all fillers up to the present time have been found lacking in some respect or other. In other words, that is, although with the use of certain types of fillers, such as high reinforcing silica fillers in certain polysiloxane compositions, were superior in properties to other known polysiloxane compositions, these superior polysiloxane compositions were still not found to meet all the performance requirements or found not to have as high performance values as would be desired.
As mentioned previously, asbestos was considered for use as a filler in polysiloxane compositions. However, even with the use of asbestos as a filler, the elastomeric polysiloxane compositions that were obtained were found to have only low tensile strength, low tear resistance, as well as not to have a high hardness.
It is one object of the present invention to produce a heat-curable polysiloxane composition with high tensile strength and exceptionally high heat resistance at high temperatures.
It is another object of the present invention to produce a heat-curable polysiloxane composition which has good flexibility at low temperatures, as well as at high temperatures.
It is yet still another object of the present invention to produce a heat-curable polysiloxane composition having asbestos fibers therein, and having a high hardness value as well as the desired elongation.
It is yet another aim of the present invention to provide a process for producing a heat-curable polysiloxane composition which has a high tensile strength, exceptionally good heat resistance, the desired flexibility within a broad temperature range, a high hardness and the desired per cent of elongation.
These and other objects of the present invention are accomplished in accordance with the polysiloxane composition and the process for producing this polysiloxane composition set forth below.